This invention relates generally to the field of micro-electrical-mechanical systems (MEMS), and in particular, to improved MEMS devices and methods for their use with fiber-optic communications systems.
The Internet and data communications are causing an explosion in the global demand for bandwidth. Fiber optic telecommunications systems are currently deploying a relatively new technology called dense wavelength division multiplexing (DWDM) to expand the capacity of new and existing optical fiber systems to help satisfy this demand. In DWDM, multiple wavelengths of light simultaneously transport information through a single optical fiber. Each wavelength operates as an individual channel carrying a stream of data. The carrying capacity of a fiber is multiplied by the number of DWDM channels used. Today DWDM systems employing up to 80 channels are available from multiple manufacturers, with more promised in the future.
In all telecommunication networks, there is the need to connect individual channels (or circuits) to individual destination points, such as an end customer or to another network. Systems that perform these functions are called cross-connects. Additionally, there is the need to add or drop particular channels at an intermediate point. Systems that perform these functions are called add-drop multiplexers (ADMs). All of these networking functions are currently performed by electronicsxe2x80x94typically an electronic SONET/SDH system. However SONET/SDH systems are designed to process only a single optical channel. Multi-wavelength systems would require multiple SONET/SDH systems operating in parallel to process the many optical channels. This makes it difficult and expensive to scale DWDM networks using SONET/SDH technology.
The alternative is an all-optical network. Optical networks designed to operate at the wavelength level are commonly called xe2x80x9cwavelength routing networksxe2x80x9d or xe2x80x9coptical transport networksxe2x80x9d (OTN). In a wavelength routing network, the individual wavelengths in a DWDM fiber must be manageable. New types of photonic network elements operating at the wavelength level are required to perform the cross-connect, ADM and other network switching functions. Two of the primary functions are optical add-drop multiplexers (OADM) and wavelength-selective cross-connects (WSXC).
In order to perform wavelength routing functions optically today, the light stream must first be de-multiplexed or filtered into its many individual wavelengths, each on an individual optical fiber. Then each individual wavelength must be directed toward its target fiber using a large array of optical switches commonly called an optical cross-connect (OXC). Finally, all of the wavelengths must be re-multiplexed before continuing on through the destination fiber. This compound process is complex, very expensive, decreases system reliability and complicates system management. The OXC in particular is a technical challenge. A typical 40-80 channel DWDM system will require thousands of switches to fully cross-connect all the wavelengths. Conventional opto-mechanical switches providing acceptable optical specifications are too big, expensive and unreliable for widespread deployment.
In recent years, micro-electrical-mechanical systems (MEMS) have been considered for performing functions associated with the OXC. Such MEMS devices are desirable because they may be constructed with considerable versatility despite their very small size. In a variety of applications, MEMS component structures may be fabricated to move in such a fashion that there is a risk of stiction between that component structure and some other aspect of the system. One such example of a MEMS component structure is a micromirror, which is generally configured to reflect light from two positions. Such micromirrors find numerous applications, including as parts of optical switches, display devices, and signal modulators, among others.
In many applications, such as may be used in fiber-optics applications, such MEMS-based devices may include hundreds or even thousands of micromirrors arranged as an array. Within such an array, each of the micromirrors should be accurately aligned with both a target and a source. Such alignment is generally complex and typically involves fixing the location of the MEMS device relative to a number of sources and targets. If any of the micromirrors is not positioned correctly in the alignment process and/or the MEMS device is moved from the aligned position, the MEMS device will not function properly.
In part to reduce the complexity of alignment, some MEMS devices provide for individual movement of each of the micromirrors. An example is provided in FIGS. 1A-1C illustrating a particular MEMS micromirror structure that may take one of three positions. Each micromirror 116 is mounted on a base 112 that is connected by a pivot 108 to an underlying base layer 104. Movement of an individual micromirror 116 is controlled by energizing actuators 124a and/or 124b disposed underneath base 112 on opposite sides of pivot 108. Hard stops 120a and 120b are provided to limit movement of base 112. Energizing left actuator 124a causes micromirror 116 to tilt on pivot 108 towards the left side until one edge of base 112 contacts left hard stop 120a, as shown in FIG. 1A. In such a tilted position, a restoring force 150, illustrated as a direction arrow, is created in opposition to forces created when left actuator 124a is energized. The restoring force arises due the pivot acting as a flexure such as a torsion beam or a cantilever beam. A hinged pivot would have no restoring force.
Alternatively, right actuator 124b may be energized to cause the micromirror 116 to tilt in the opposite direction, as shown in FIG. 1B. In such a tilted position, a restoring force 160, illustrated as a direction arrow, is created in opposition to forces created when right actuator 124b is energized. When both actuators 124 are de-energized, as shown in FIG. 1C, restoring forces 150, 160 cause micromirror 116 to assume a horizontal static position. Thus, micromirror 116 may be moved to any of three positions. This ability to move micromirror 116 provides a degree of flexibility useful in aligning (including aligning, pointing, and/or steering) the MEMS device, however, alignment complexity remains significant.
In certain applications, once the micromirror is moved to the proper position, it may remain in that position for ten years or more. Thus, for example, one side of an individual micromirror may remain in contact with the hard stop for extended periods. Maintaining such contact increases the incidence of dormancy related stiction. Such stiction results in the micromirror remaining in a tilted position after the actuators are de-energized. Some theorize that stiction is a result of molecule and/or charge build-up at the junction between the micromirror and the hard stop. For example, it has been demonstrated that capillary forces due to an accumulation of H2O molecules at the junction increases the incidence of stiction.
Thus, one solution to overcome stiction is to package the MEMS device in a hermetic or inert environment. Such an environment reduces the possibility of molecule accumulation at the junction. However, such packaging is costly and prone to failure where seals break or are not properly formed. Further, such packaging is incompatible with many types of MEMS devices. In addition, such packaging does not reduce stiction related to charge build-up at the junction.
In xe2x80x9cUltrasonic Actuation for MEMS Dormancy-Related Stiction Reductionxe2x80x9d, Proceedings of SPIE Vol. 4180 (2000), Ville Kaajakari et al. describe a system for overcoming both molecule and charge related stiction. The system operates by periodically vibrating an entire MEMS device to release stiction forces. In this way, the stiction forces are overcome. While there is evidence that vibrating the entire MEMS device can overcome stiction, such vibration causes temporary or even permanent misalignment of the device. Thus, freeing an individual micromirror often requires a costly re-alignment procedure. Even where the device is not permanently misaligned by the vibration, it is temporarily dysfunctional while the vibration is occurring.
Thus, there exists a need in the art for systems and methods for increasing alignment flexibility of MEMS devices and for overcoming stiction in MEMS devices without causing misalignment.
The present invention provides improved MEMS devices and methods of using and making the same. Some embodiments are particularly adapted for use with optical networks. Thus, some embodiments of the present invention include a structural plate comprising a micromirror. For example, the present invention may be used with the exemplary wavelength routers described in co-pending U.S. patent application Ser. No. 09/442,061, filed Nov. 16, 1999, now U.S. Pat. No. 6,501,877, the complete disclosure of which is herein incorporated by reference.
Some embodiments of the invention comprise an electromechanical machine which includes a base layer. At least one actuator and two structural plates are disposed over the base layer. The two structural plates are supported by pivots and are above the actuator. Activation of the actuator causes a side of one structural plate to contact the base layer or a structure thereon. In this position, a side of the other structural plate is deflected toward the base layer.
Other embodiments of the invention further comprise a second actuator disposed over the base layer. Activation of the first and the second actuators causes the second structural plate to move toward the first structural plate, thus causing the first structural plate to contact the second structural plate.
Some embodiments of the present invention comprise a method for moving plates in an electromechanical device between a plurality of stop positions. The methods comprise moving a first plate to select a stop position for the second plate, wherein the stop position is one of a plurality of possible stop positions. The second plate is moved until it contacts the selected stop position.
Other embodiments comprise a wavelength router for receiving light having a plurality of spectral bands at an input port and directing a subset of the spectral bands to one of a plurality of output ports. The wavelength router comprises a free-space optical train disposed between the input port and the output ports, wherein the optical train provides at least one path for routing the subset of the spectral bands, the optical train including a dispersive element disposed to intercept light traveling from the input port and a routing mechanism having at least one dynamically configurable routing element to direct a given spectral band to different output ports depending on a state of the dynamically configurable routing element. The dynamically configurable routing element comprises a micromirror assembly disposed over a base layer and a control member disposed adjacent the micromirror assembly. The control member is moveable to select a movement limit for the micromirror assembly.
Yet other embodiments of the invention comprise computer readable code for execution by a microprocessor. When executed by a microprocessor, the computer readable code causes the microprocessor to configure plates in a micromirror device. Configuring the plates includes moving a first plate to select a stop position for a second plate with the stop position selected from a plurality of stop positions, and moving the second plate to the selected stop position.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.